Dog clutches of this kind have to perform many, often counteracting tasks, as indicated in the article in MOTORRUNDSCHAU 20/64.
For instance, the synchronization of the coupling parts (gear and synchronizer body) running at different rotational speeds has to be achieved quickly and with a minimum of shift forces. Also, after synchronization, it has to insure an easy decoupling and a quick form-locking coupling, so that there is overall, a quicker and smoother functioning of the shifting process.
These requirements have to be satisfied with functionally simple means, which are also inexpensively manufactured.
One way to approach these objects is by increasing the number of friction surfaces, in order to obtain higher friction torque with reduced shift forces.
From the WO No. 85/01 998, a device is known wherein the friction surfaces of the frictional connection assigned to the gear are connected to the gear only in the active position. The synchronizer rings, arranged one over the other in radial direction, are also connected only to the synchronizer body with circumferential play and for the purpose of locking.
The German open application No. 14 50 825 shows that synchronizer rings can be connected to each other by gear couplings.
Such devices, due to the considerable increase of the friction surfaces, lead to a more rapid speed equilization with commensurate shift forces without overloading the friction surfaces, when compared to a simple friction pairing. However, the decoupling is not satisfactory, as a result of the blunt locking surfaces, which are required for secure locking and because the manufacturing efforts for such a device are too high.